Vacuum-tube amplifier circuit



Aug. 2'7, 1935. v. J. FABIAN 2,012,381

VACUUM TUBE AMPLIFIER CIRCUIT Filed Jan. 8, 1931 Patented Aug. 27, 1935 2,012,381 VACUUM-TUBE AMPLIFIER CIRCUIT Vincent J. Fabian, Washington, D. 0. Application January 8, 1931,;Serial No. 507,497

16 Claims. (01. ire-171i This invention relates to amplifier, or repeater, circuits employing space charge tubes, and concerns particularly an extremely efiicient circuit in which successive tubes are directly coupled.

in cascade in such manner as to produce without distortion progressive full wave amplification of a magnitude determinable by the amplifying factors of the successive tubes, said circuit at the same time being non-resonant, non-microphonic and equally responsive to an unlimited range of frequencies of both large and small amplitude. Having these desirable characteristics, it is equal- 1y adaptable to radio receivers and transmitters, telephone and telegraph systems, and, in i fact, to any use wherein there is need of a sensitive amplifier capable of exacting, undistorted an highly amplified output.

summarily stated, the invention contemplates a circuit including direct tube coupling such that successive tubes will progressively constitute the sole amplifying agency to which successively signal energies of any frequency and amplitude will freely pass at zero impedance without current or voltage lag and be amplified therein to substantially the full amplifying ability of the tubes, the coupling being so related to the rest of the circuit that a state of balance will exist in the absence of signals during which the coupling will offer infinite impedance to or neutralize circuit 30,2 potentials impressed thereon with a resulting free condition of the tube grids, the balance being subject to control of signal energies so that when signals are impressed upon the circuit the balance will be broken and the grids impressed with a variable bias simultaneously with the determinable in amount by the amplitude of the signals. As more fully pointed out, Where I use the term balance I have reference to a condition such that there is no flow of current either in the tube or the external circuit.

To obtain full and undistorted amplification limited only by the amplifying ability of the tubes themselves, I provide a direct path or coupling circuit between the plate and grid of adjacent tubes, said coupling circuit having no amplifying characteristic itself and being such as to minimize current and voltage lags and losses. Such coupling circuit is preferably untuned and of itself so balanced as to be equally responsive to all F signal frequencies, and has the characteristic of oiiering zero impedance to signal energy, thus allowing full wave amplification by the tubes to their maximum ability, while offering, under a condition of balance hereafter to be discussed,

; substantially infinite impedance to direct potentials which would otherwise undesirably bias the portant feature of this invention to eliminate the customary fixed grid bias which necessarily limits the grid swing and hence the amplificationavailable from the tubes, and to utilize free flexible grids upon which a bias controlled byandvari able as to quantity and time in accordance with the amplitude of the signals to be amplified is impressed. To this end the circuit including the tube couplings is so constituted as to be put in a condition of balance in the absence of signals, at which time the couplings will offer infinite impedance to the passage of circuit potentials from the external circuit to the grids of the tubes, and' neutralize any charges thereon arising due to conditions within the tubes, so that undersuch conditions the grids will be maintained neutral and unbiased, and the plate current will be zero. Speaking generally, such conditions are brought about by providing in the external circuit instru mentalities which will be productive of potential drop andbalance the internal impedance of the respective tubes, and connecting successive'tubes by paths freely conductive to signal energies but capable of offering under prescribed conditions of circuit balance infinite impedance to potentials which would otherwise charge the grids of the tubes. Such balanceis arranged tobe subject to the control of impressed signals so that when signal energy is -present the balance, so far'a's it maintains the grids neutral, is broken, and a bias controlled by the amplitude of the signals and having its source preferably in the plate potential of the preceding tube is' impre'ssed upon the grids of the several tubes.

Having now discussed generally the objects and essential characteristics of my inventiom'I' will now describe the same in detail in connec tion with the accompanying drawing illustrating several embodiments thereof, particular ref erence being had to the nature and function of the elements and their arrangement in a circuit calculated to produce the desired results and the operation of the whole circuit as it is now understood. I

In the drawing: Fig. 1 is a diagrammatic illustration of the fundamentalcircuit constituting the present invention; i

Fig. 2 isa similar view illustrating the embodiment of the fundamental circuit of Fig. 1 in a multi-stage amplifier; and

Fig. 3 is a view showing the embodiment of my invention in a circuit somewhat similar to the socalled push-pull amplifier.

Fig. 4 is a curve diagram showing the relation between plate current and grid voltage and between grid current and voltage indicating the characteristics of my amplifier circuit.

The action of a tube as used in. my amplifier circuit will be best understood from the characteristic curves shown in Fig. 4. These curves show the relation between plate current and grid current and between grid and. plate voltage. It will be noted that the axis y-y represents the zero of plate and grid currents, while the axis x-x represents the zero of grid potential.

The dotted portions of the grid current and plate current curves illustrate the relation between such currents as the grid voltage increases from negative to positive, it being noted that as the grid potential increases the plate current risesuntil it reaches a: maximum. The conventionalramplifier operates in such manner that the plate current and gridcurrent vary in amplitude as illustrated, the rangein variation being represented by the portions of the curves between aand'B.

It may be noted, generally, from the curves that as'the grid potential increases the plate currentincreases until a maximum is reached. The

conditions and changes in plate current and grid current in my amplifier circuit are entirely different. While in the ordinary circuit there will willbe no substantial change from the maximum values of plate current and grid current from that indicated by the curves as the grid potential is further increased, in my circuit as the grid potential is further increased the grid 1 current becomes less negative and the plate currentless positive until they meetat the point 0. As the. grid potential is further increased, the grid; current becomes increasingly positive and the plate current becomes increasingly negative until; maximum values are reached, and from the maximum points they bend upwardly to again; meet the y-y axis. In otherwords, with my amplifier conditions are such that the plate current changes from a maximum'positiveto' a maximum negative value, while the grid current changesfrom a maximum negative to a maximum positive value-in accordance with variations in*grid-=p0tentia1-. 'Under such circumstances. I am;- able to. establish an initial state of balance as represented by the point 0 on the curves in thefdiagram; under ,whichthere is no current flow either inthe external circuit or in the tube inqthe-absence of signals. When signals are impressed thebalance of the circuitis broken with resulting variation in grid potential which 1111- creases or diminishes in accordance with: the signand amplitude of the signal. The plate current,.: representing theamplifiedoutput of the tube, undergoes-a? correspondingchange,=increas'-' ing' positively or negatively within the range indicated between the pointsC and'D;

'In:Fig 1"I.havediagrammatically illustrated a three-element; space charge tube .and thefundamental circuit constituting the-present invention associated therewith in such manner as to function as above described. It isappropriate particularly: in connection'with this fundamental ci-rcuiteto point out that while-my essential pur pose is to provide a circuit adapted to amplification. at. audiofrequencies, the circuit, for reasons not entirely understood, has been found to function as a radio frequency amplifier, and, so far as I have been able to ascertain, does not require a separate detector or rectifying tube in the supporting circuit. In this latter connection it may be noted in passing that the input or coupling circuits of the tubes is somewhat like the well known detector circuit including as it does a condenser C and resistance Rc arranged in shunt in the grid lead. While possibly this part of my circuit may act like the gridleak and condenser in a detector circuit so as to permit rectification within the tubes, it has, however, different functions hereafter to be discussed. Furthermore, it is my present belief that the circuit is nondiscriminatory between rectified and unrectified signals since it may be connected directly to a tuning circuit and antenna and produce an amplified and high quality of rectified signal in an output device.

Considering now the physical aspects of my fundamental circuit as depicted in Fig. 1, a sourceof signal energy which may constitute a previousintermediate tuned or untuned, antenna or space charge tube coupled, circuit, inductively, capacitatively or impedance coupled to the present circuit,

ity connection is well known, and the resistance by itself would be inefficient, their arrangement in a shunt, when properly balanced,v will, since untuned and non-productive of distortion, render the path to the grid freely conductive to signal energies of all useful frequencies at zero impedance without current or voltage lag or loss. By reason of the inherent nature of the capacity, which may be preferably a fixed or variable condenser, there will be a 180 degree turnover of the signal so as to be input on the grid of the tube at the opposite sign.

The values of C and He may be variedwithin relatively large limits, depending primarily on the mutual conductance of the particular type of space charge tube used and the potential associated with the source-of signal side of the shunt. The coupling shunt serves a double purpose, and the balance of the resistance and capacity is calculated, when a signal is present, to permit sustained oscillations whatever the frequency of the signal, and transfer the signal energy to the grid of the tube.

It will, of course, be appreciated that a direct path including the above elements has no appreciable, if any, amplifying ability and thus eliminates one source of distortion and enables space charge tubes to constitute the sole amplifying agency in the circuit. the several functions of the resistance-capacity shunt coupling circuit are important. In order to properly understand such functions, it is first necessary to consider other phases of the circuit, with particular reference to their relation to the coupling circuit and the utilization of a flexible grid bias controlled by the amplitude of impressed signals in such'manner as will enable the space charge tubes to operate tosubstantially their full amplifying factors.

Referring still to Fig. 1, it will be noted that an impedance R-l is included in the plate circuit of the tube giving a potential dropbetween the source of potential and the plate with resulting benefits to tube operating efficiency for reasons well known, since this in itself isa com- In this latter connection mon expedient. In the present circuit R| should be ofsuch value as to substantially balancethe internal impedance of the particular tube in relation to the value of potential source used. Such impedance should be a pure resistance since it is desirable for the same to be noninductive, but it will be understood that a reactance can be employed with fair results.

It will be noted from an'inspection of the figure that no source of fixed grid bias is used, nor is the grid of the tube directly or indirectly connected to the filament or plate potential return circuit since its only physical connection is with the plate side of the plate potential. Such an arrangement thereby in effect isolates the grid from the electrical circuit and, in fact, from all external-relation with the plate and filament. It is of'course necessary to have a grid bias of some sort to enable the tube to operate. With the present arrangement except for charges arising within the tube the grid is substantially free and neutral. By means now to be described, I am able to'maintain the grid in such state in the absence of signals and to neutralize charges capacitatively produced in the tube, and likewise effect a variable bias suitable for a particular signal amplitude when such is impressed upon the tube circuit.

In addition to the impedance R-l in the plate circuit of the tube I providean additional impedance R likewise connected to-the source of potential having a value considerably smaller than that of R-I so that the effective grid bias will be less than the plate potential and produce a substantial plate effect. If the source of signal energy be a previous space charge tube, as in Fig.2, such impedance will constitute a source of potential drop for the plate of such tube. However, in its present connection its particular purpose is to provide a source of grid bias for the amplifying tube and to this end is connected to the signal end of the grid coupling circuit. Being also associated with the source of signal energy, the presence of which will break down the balance of the coupling circuit maintaining the grid in a neutral state and permit the grid vto receive a bias, it constitutes a source of bias controllable by the amplitude of the signal impressed.

Considering now the resistance capacity coupling in relation to the above-describedsource of grid bias, it will be appreciated that by a proper value of the shunt resistance Rc ,withrelation to R, R| and the source of potential, a balanced circuitcan be formed such as will prevent the biasing of the grid except as desired. Thus, by determining R 50 as to block passage of potential bias to the grid, or, in other'words, make R0 of a value which in the absence of signals will infinitely impede the potential available after the drop across R, the grid will be maintained neutral andunbiased. Furthermore, any parasitic oscillations or charges arising at the grid will be neutralized by reason of the opposite polarity existing at the respective ends of the shunt and the tendency for a current flow of one sign towards the grid equal to and neutralizing a how of current of opposite sign from the grid.

By the above arrangement for grid bias, the grid coupling is made to balance by proper valuation of the several impedances so as to maintain the grid'neutral when no signals are present. However, as soon as a signal sine wave of either sign is impressed upon the coupling the balance is broken and a bias impressed upon the grid corresponding in amount to the amplitude of the signal, thus effecting a grid swing such as will produce a plate effect determinable by the amplification factor of the tube, since the signal willunbalance the circuit and cause a grid bias appropriate for the greatest amplification of the particular amplitude of such signal. The grid swing will be limited substantially only by the potential variation at the signal end of the coupling, and may be either positive or negative relative to the plate and produce a plate effect, thus allowing full wave amplification.

Considering the arrangement of the circuit in connection with the characteristic curves shown in Fig. 4, it will be understood that the portion of the circuit including the resistances R|, R, Rc is in shunt with the internal resistance. of the tube between the grid and plate, and serves to bring the circuit to a neutral balanced state represented by the point 0 on the curves, at which time the plate and grid currents are zero. The resistance capacity shunt Rc-C serves to maintain such balance by neutralizing any charges on the grid arising within the tube. When an oscillatory current to be amplified is impressed upon the circuit over the path including the RcC shunt the balance is broken under the resulting change in grid potential. The plate current increases either positively or negatively in accordance with the sign and amplitude of the impressed current within the range indicated between the points C and D.

With reference to Fig. 2, wherein I have shown a two tube amplifier, having the characteristics above described connected with a source of signal energy constituting a preceding space charge tube. It will be noted that R which furnishes thesource of bias for the grid of the first amplifying tube constitutes a plate impedance for the input tube, and of course the value of the bias will be determined by the variations in plate effect of the input tube. R should be considerably smaller in value than R| so that the effective grid bias available across the resistance-capacity shunt coupling Rc-C will be substantially less than the plate potential of the first amplifying tube; otherwise the tube cannot be productive of material amplification. The second amplifying tube is connected to the plate of the first by a coupling Rc--l C|, the available grid bias being determined by the potential drop across impedance R-|. In the last amplifier, it should be noted that no resistance, or only a small resistance, is required since an output device which is preferably connected in the plate circuit has sufficient resistance to produce a potential drop which will insure proper operation of the tube.

In the circuit shown in Fig. 2, utilizing about 180 volts potential and average tubes with amplification factors of about 3 or 4, the values of the capacities and impedances may be asv follows: R-25,000 ohms, R-l 50,000 ohms, R0 10 megohms, Rcl 5 megohms, C and 0-! .01 mf. These values are only illustrative, and other values within a considerable range may be used for the same type tubes and source of plate potential. It will of course be appreciated that the values of the shunt resistance Rc will increase with successive tube amplifiers as the plate impedance R decreases. This is due to the fact that when R is smaller there will be a higher available grid bias which must be balanced by Re to give a free grid when no signal is present.

Referring to Fig. 3, I have therein shown the adaptation of my fundamental circuit to a compound, multi-stage amplifier giving undistorted current and voltage amplification. It is in effect merely an arrangement of two amplifiers of the type shown in Fig. 2 in shunt and connected to an output transformer and has'the same principle of operation. It resembles the well known pushpull circuit, but functions diiferently therefrom and is capable of a greater power output. Since by reason of the peculiar nature of my fundamental circuit each tube gives full wave amplification, a signal will be simultaneously input to both branches of the circuit instead of successively with 180 degree phase difference. Thus, the amplified. signals from both branches are simul-- taneously fed to the output transformer and voltage as well as current amplification isobtained.

I claim: 7

1. In a space charge tube circuit a source of signal energy, a tube having an input and an output, said input being connected to said source, an impedance path between the source and input, an impedance connecting with the output, said impedances constituting a shunt between the input and output adapted to prevent the flow of current therebetween in the absence of signal energy and a source of potential connected to the inputand output through saidimpedances.

2. In a space charge tube circuit a source of signal energy, a tube having an input and an output, a resistance-capacity shunt connecting the source and input, impedances'connecting the input and the output, said impedances constituting ashunt between the input and output adapted to'prevent the flow of current therebetween in the absence of signal energy and a source of potential connected to the input and output through said impedances.

3. In a space charge tube circuit a source of signal energy, a tube having an input and-an output, a path including a resistance-capacity shunt connecting said source to said input, a source of potential, impedances connecting said source of potential with the output and the signal side of said shunt, said impedances and the shunt resistance constituting a shunt circuit between the input and output and being of such character as to provide a balance with the internal impedance of the tube characterized by the absence of current flow in the absence of signals and being such as to be thrown out of balance when signals are impressed to provide a bias variable according to changes in amplitude of said signals.

4. A multi-stage space charge tube circuit including tubes each having an input and an output, direct paths between the input and the output of adjacent tubes, said paths including a resistance-capacity shunt, a source of grid bias for the tube inputs and means controlled by signals from the signal source for varying said bias.

5. A resistance coupled amplifier including a plurality of amplifier tubes in cascade, a source of anode potential, a common lead between the positive side of said source of potential and each of the tube anodes, a coupling impedance be tween each anode and said common lead, a direct isolated signal path between the anode of each preceding tube and the input grid of each succeeding tube and serving to supply the'input grids with a variable bias from said common lead, said paths each incorporating an impedance adapted to produce a drop in the grid biasing potential between the anodes and grids and also incorporating a coupling capacitance.

6. In a multi-stage space chargetube circuit comprising a series of tubes each'having an input and an output, direct couplings including a resistance-capacity shunt connecting the input and output of'adjacent tubes, a source of potential, impedances connecting said source to the tube inputs and outputs-said impedances and shunt resistancesconstituting a shunt between the input and output of a character such that no current will flow and in the absence of signals the tube inputs will remain uncharged, said balance being adapted to be broken in the presence of signals so that a bias from said source of potentialdeterminable in amount according to variations in the signal will be impressed upon the tube inputs. I

7.. In a multi-stage space charge tube circuit, a source of signal energy, a series of tubes each having an input and an output, paths including aresistance-capacity shunt connecting the input and=output of adjacent tubes, a source of potential, and impedances connecting the positive side of said source to the tube outputs and said paths.

-8. In a multi-stage space chargetube circuit, a-sourceof potential, a series of tubes each havingan input and anoutput, a path including a resistance-capacity shunt connecting the input and output of adjacent tubes, a source of potential, and impedances connecting said source to the tube outputs and said paths, said impedances and said'resistance-capacity shunts constituting a shunt between the input and-output of'a character such that no currentwillflow and the tube inputs will remain uncharged in the absence of signals, said balance being adapted to be broken in the presence of signals to permit a variable bias determinable by signal variations to be imposed upon the tube inputs through said paths.

9. A multi-stage space charge tube circuit including a source of potential and a series of tubes each having an input and an output, direct couplings between the input and output of adjacent tubes adapted .to freely pass signals .from the tube outputs to the adjacenttube inputs,'a source of potential the positive side of which is connected tothe outputs and said paths through means adapted to maintain an electrical balance and prevent the passage of potential to the tube inputs through said paths, said means including an impedance shunt between the output and input of a character such that in the absence of signals no current .will flow, said balance being so controlled by the signals that the introduction of a signal to said paths will break the balance and permit a bias to be impressed-upon the tube inputs determinable in amount according to variations in signal amplitude. f

10. A multi-stage space charge tube circuit including a source of signal energyand a series .of tubes each having an input andan output, direct couplings between the input and output of adjacent tubes adapted to freely pass signals from the tube outputsto the adjacent tubeinputs, a source of ,potential the positive side-of which is connected to the outputs and said. paths through means adapted to maintain an electrical balance and prevent the passage of potential to the tube inputs through said paths, said means including an impedance shunt between the out- .put and input of a character such that in the absence ofsignals no current will fiow, means including aresistance-capacity shunt in said paths adapted tcpermit the passage of signals to the tube input and neutralize charges on the input arising within the tubes; the balance being so controlled .by the signals that the introduction of a signal to said paths will break the balance and permit a bias to be impressed upon the tube inputs determinable in amount according to variations in signal amplitude.

11. In a space charge tube circuit, a source'of signal energy, a tube having an input and an output, a path including an impedance-capacity shunt connecting said source to said input, an impedance path between the signal side of the input and the output, said impedance path and said impedance-capacity path shunting and balancing the internal resistance of the tube, and a source of potential connecting with the output through the impedance path.

12; The combination with a space charge tube having a cathode, an anode and a grid, of a source of signal energy, an input circuit including an impedance-capacity shunt connecting said source and the grid, an impedance path between the signal side of the input circuit and the anode, and a source of potential connecting with the anode through the impedance path, the impedance path and the impedance-capacity input having no connection with the filament save through the source of potential and shunting and balancing the internal resistance of the tube between the grid and the anode, so that a bias will be imposed upon the grid by the source of potential variable in amount in accordance with and under control of incoming signals.

13. In a space charge tube circuit, a plurality of tubes, each having a cathode, an anode and a grid, a direct path including an impedance between the anodes and grids of adjacent tubes, an impedance path between the grid and anode of the respective tubes, a source of potential, an impedance path connecting the source of potential to the tube anodes, the grids of the tubes having no direct connection with the cathodes save through the source of potential.

14. In a space charge tube circuit, a plurality of tubes, each having a cathode, an anode, a direct path including an impedance between the anodes and grids of adjacent tubes, a source of potential, impedance paths connecting the source of potential to the anode of each tube and serving to provide a bias for the grids of the tubes through the direct paths between the anodes and grids of adjacent tubes, the grids of the tubes having direct connection only with the anodes so that the bias will be derived solely from the positive side of the source of potential.

15. A cascade space charge tube circuit including a plurality of tubes each having an input grid, an output plate and a source of electron emission, at common source of potential supplying the several tube plates, a direct coupling between successive tubes comprising an intermediate circuit which provides a direct path freely conductive to signal energy between the output of one tube and the input or" the following tube, said intermediate circuit being connected to and supplying to the tube inputs a grid bias from the output side of the source of plate potential and being isolated from the negative side of the source of plate potential and from the tube sources of electron emission.

16. In a space charge tube circuit, a plurality of tubes, each having an input grid, an output plate and a source of electron emission, a source of potential supplying the tube plates, a circuit intermediate successive tubes serving to transfer signal energy and also to supply a grid bias, said intermediate circuit including a portion pro-- viding a direct path between the plate of the preceding tube and the grid of the following tube which permits the free fiow of signal energy therebetween, said direct path and the other portions of the intermediate circuit being isolated from the negative side of the source of plate potential and from the tube sources of electron emission and being so connected with the output side of the source of plate potential as to supply therefrom a bias to said following tube grid along the signal path portion of the intermediate circuit; the intermediate circuit forming a shunt across the input and plate of said following tube and including means which in the absence of signal energy will cause the circuit to offer an effective impedance approximately balancing the internal resistance of the tube and blocking the passage of bias to said tube grid but so constituted that the passage of signal energy to said tube across the direct path portion of the circuit will destroy the circuit balance and cause a bias to be impressed on the tube grid to a value determined by and variable with the intensity of the signal energy.

VINCENT J. FABIAN. 

